Numerous previous investigators have described methods for chemically passivating thermoplastic resins and for protecting certain thermoplastic resins from ultraviolet attack by additives that absorb incident radiation. Rauhut 3,974,368, Aug. 10, 1976, describes passivating polyethylene surfaces using silanes, e.g., dimethydichlorosilane and the like. Uhl No. 3,810,775, May 14, 1974, describes a process for making water repellant fiberous materials by applying a copolymer of ethylene and a vinyl halosilane or vinyl alkoxy silane. Anyos and Moyer No. 3,423,483, Jan. 21, 1969, describes production of a fluorescent polymer using polybenzoxazole units.
A modern process particularly suitable and often employed for the production of thermoplastic aromatic polycarbonates consists of reacting phosgene with suitable bisphenols in an aqueous solution or suspension of alkali or alkaline earth metal salts. The polycarbonates that are obtained are high molecular weight linear chains of repeating units "X" and "Y"; distributed more or less at random; ranging from high ratios of "x"/"y" to high ratios of "y"/"x" having end groups of [HO-] and ##STR1## where "X" is ##STR2## and Y is all more fully described in Wulff, Schnell, and Bottenbruch No. 3,422,065, Jan. 14, 1969, the entire disclosure of which is hereby incorporated by reference and relied upon.
The linear chain polycarbonate thus produced can be cross-linked in the presence of oxygen or free radical forming catalysts such as dibenzyl peroxide and dicumyl peroxide with the resultant cross-linked polymer being much more insoluble than the linear polymer to high temperature steam. A preferred bisphenol intermediate for the cross-linked polycarbonate is "Bisphenol Cy" 1,1-bis(4 hydroxyphenyl) cyclododecane, but the resulting cross-linked resin presents substantially more difficult processing parameters than the linear polycarbonate.
Examination of the stepwise attack on linear polycarbonates by strong bases offers an alternative to cross-linking for purposes of producing stress corrosion resistance while retaining favorable processing characteristics. Strong bases, and to a lesser extent, water, at temperatures ranging from 140.degree. F. to 200.degree. F. produce stress corrosion responses in linear polycarbonates. This response is enhanced by the presence of residual or induced stress and is increased with increasing temperature. Stress corrosion responses characterized by cracking, pitting, loss of ductility, and weight loss by mass fall out of exposed areas may be noted as attacks following stepwise polymer bond breakage. Chemical reactions first producing bond breakage are most likely those involving relatively energy-rich, vulnerable, end group sites on the linear polymer chain. The compounds formed by reacting the energy-rich units with stress corrodant constituents are more voluminous than the reactant and create a stress field sufficient to cause macroscopic fractures. The corrosion velocity or rate of attack is a function of the rate of supply of an environmental reactant, and the amount of energy or stress available. Corrodant diffusion rates, surface to volume effects, and crack opening by external forces thus play important roles in defining attack velocities.
The importance of the end group's chemical reactivity may be noticed by comparing preparation of thermoplastic polycarbonates with the preparation of a thermoplastic polysulfone. Polysulfone is prepared by reacting bisphenol A and 4,4-dichlorodiphenyl sulfone with potassium hydroxide in dimethyl sulfoxide. The characteristic polysulfone resin ##STR3## offers no site of high unit energy and minimal potential for chemical reactions causing pressure generating increased volume products. It is necessary to remove all but slight traces of water before polymerization to prevent hydrolysis of the dihydric phenol salt, and subsequent formation of the monosodium salt of 4-chloro-4-hydroxydiphenol sulfone. End groups of [HO-] are absent.
In this connection consider the most common polycarbonate polyester resin.
Polycarbonate polyesters produced from bisphenol A are characterized by units of: ##STR4##
Polycarbonate resins may be slowly produced by a non-catalyzed condensation reaction between bisphenol A and phosgene. ##STR5## where n is the degree of polymerization.
This reaction may be greatly accelerated by basic catalysts. With basic catalyst acceleration, the reaction is assumed to be: ##STR6##
Where M is typically Li, Na, or K, obtained from a salt solution in aqueous medium or from salt or hydride dissolution in the fused bisphenol A resin.
Traces of the metal organic bisphenol, the catalytic salt, the hydroxyl end groups, and unreacted bisphenol in the polycarbonate resin may cause stress corrosion through the following typical stress producing reactions. ##STR7##
Exceptionally high proton mobilities in certain phases such as ice compared to water are illustrative of proton transfer along the hydrogen bond. Protons released by reactions with polycarbonate constituents also diffuse, forming hydroxyl and hydronium stress producing radicals. ##STR8##
Polycarbonate resins may also be produced by catalytic synthesis of bisphenol A and carbon monoxide: ##STR9##
This reaction is promoted by first dissolving the bisphenol A in a suitable solvent such as tetrahydrofuran (THF) and then activating it with hydrogen. Activated hydrogen is introduced to the bisphenol A through the high shear path of reactor 300 in FIG. 5 (described later) to produce the following activated intermediate: ##STR10##
The activated intermediate is then immediately reacted with carbon monoxide to produce polycarbonate. ##STR11##
This linear molecule may be grown as large as desired in the form: ##STR12##